The present invention relates generally to electrical generators that can provide backup electrical power to an electrical system or that can be used as a stand-alone power supply for powered accessories, such as power tools. More particularly, the present invention is directed to an apparatus for selectively disabling outputs of an electrical generator based on the type of load placed on the electrical generator.
In today's electrical supply systems, there are occasions when alternate sources of electrical power are necessary or desirable. For example, the capability of switching from utility power to emergency generator power is extremely important for many businesses, hospitals and industries, as well as residential dwellings. Engine driven electrical generators are commonly used to provide backup or emergency electrical power in those instances when utility power is interrupted. Additionally, portable electrical generators allow electrical power to be provided at locations where utility power is not available.
Electrical generators can generally be categorized as bonded neutral generators or floating neutral generators. With a bonded neutral electrical generator, the neutral conductor and the safety ground conductor of the electrical generator are internally tied or “bonded” together. For floating neutral generators, on the other hand, the neutral conductor and the safety ground conductor are not tied together. Bonded neutral generators are most commonly used on construction sites and are popular among the professional trades. Floating neutral generators are typically used by homeowners either to provide backup power to electrical loads through the wiring system of the home or building or to power cord connected devices.
Portable electrical generators will typically have a dedicated receptacle or socket that is designed for larger loads and to engage with a power cord that is electrically coupled either to the electrical system of the building, typically through a transfer switch and panel, or to some other distribution system such as a distribution box for a number of cord connected devices. For most portable electrical generators, a multipole locking receptacle is used to engage and lock the male end of the power cord to prevent accidental disconnection of the power cord from the electrical generator. Via the interconnection of the male end of the power cord with the multipole locking receptacle or socket, the live conductors of the electrical generator will be electrically connected to appropriate poles of the transfer switch and panel. When connecting a floating neutral generator to a building's wiring system as a non separately derived system, the neutral conductor will be electrically connected to the neutral bus of the service entrance or main panel generally through the transfer panel and the ground conductor will be connected to the ground bus of the service entrance or main panel also generally via the transfer panel. Thus, the electrical generator is “grounded” via electrical connection with the ground bus of the service entrance or main panel.
In a non-separately derived arrangement such as that described above, when a bonded neutral electrical generator is supplying electrical power to one or more loads of the building through the building wiring system, electrical current will return via the neutral conductor of the load to the neutral bus conductor of the main electrical panel. A first portion of electrical current then flows from the neutral bus conductor of the building back to the neutral conductor of the generator, thus completing a circuit path. A remaining portion of electrical current flows from the neutral bus conductor of the building to a neutral-to-ground tie bar at the electrical panel, through the grounding bus conductor, back through the safety ground-to-neutral bonding conductor of the generator, and then through the neutral conductor of the generator, completing another circuit path. As provided in the United States National Electrical Code, Article 250, a power system should be electrically grounded in such a manner that prevents a flow of electrical current via the neutral conductor of the building back to the safety grounding conductor of the generator, in all situations except for an electrical power fault (q.v., Article 250 of the National Electrical Code). The safety grounding conductor is expected to be pristine or absent of the normal flow of electrical current, and instead is to be used to conduct electrical current safely to ground only when there is an electrical fault occurrence. Thus, the use of bonded neutral generators to supply backup electrical power to the electrical system of a home or other building requires a system that switches the neutral, know as a separately derived system. This system is more costly to buy and install.
Portable electric generators will typically have one or more duplex receptacles in addition to the multipole locking receptacle for receiving the male end of an extension cord or the power cord of an electrical device. As known in the art, the duplex receptacles allow electrical devices to be powered directly by the electrical generator rather than through a building wiring system. In most instances, the duplex receptacles are designed to receive a three-prong plug of a power cord. In this regard, each socket of the duplex receptacle is designed to receive a hot conductor, neutral conductor, and ground conductor of a conventional male end of a three-prong plug. As is known, each socket of the duplex receptacle is also capable of receiving a non-grounded plug of a power cord.
Floating neutral electrical generators are typically not grounded, i.e., not only is the neutral not connected to the safety ground but the safety ground is also not connected to earth because the generator frame is not conductively connected to earth such as through a grounding rod. As a result, when the generator is used as a stand-alone power supply or providing electrical power directly to one or more electrical devices without an electrical connection to the electrical wiring system of the building, ground fault protection is unnecessary because a path for current to flow back to the generator does not exist through the ground. In other words, ground fault protection is built into the system through isolation of the neutral wire from the ground thereby eliminating the need for duplex receptacles incorporating ground fault circuit interrupter (GFCI) devices. However, when a floating neutral generator is connected to the electrical system of a building, the generator becomes grounded through its electrical connection to the electrical system of the building. As a result, there can be a ground fault risk when a floating neutral generator is connected to the electrical system of a building. One way to reduce this risk on floating neutral generators is to equip them with duplex receptacles that include GFCI devices, which can be costly and are only needed when the generator is connected to a building.
Increasingly, the professionals and homeowners have demanded a single generator capable of being used for both construction and for backup power supply for the electrical system of a home, apartment, or other building when utility power is interrupted. They all would prefer to install the generator in a non separately derived system as this is the most common and economical installation. For a floating neutral generator to accomplish this task, costly GFCI protected duplex receptacles should be used for ground fault protection when the generator is connected to the electrical system of the home or building. Since bonded neutral generators internally bond the neutral and ground conductors, costly GFCI devices are also used to provide ground fault protection primarily in construction applications where they are now required by the NEC. Moreover, it has been found that connecting a bonded neutral generator to the electrical system of a home or building as a non separately derived system can result in “false” triggering of ground faults. That is, the flow of electrical current to the safety grounding conductor of the generator has been known to trigger a ground fault circuit interrupter at the generator. When triggered, the ground fault circuit interrupter will de-energize the live conductors of the generator and prevent the supply of electrical power to the circuits connected to the transfer equipment. To avoid this nuisance tripping of the GFCI, the consumer must un-tie the bonding of the generator neutral and the generator ground. Many consumers are hesitant to tackle such a task and, moreover, it requires the consumer to remember to retie the generator neutral and the generator ground when the electrical generator is used to power electrical equipment directly rather than through the wiring of the home or building.
The present invention provides an interlock arrangement that allows a floating neutral electrical generator having a receptacle, such as a duplex receptacle, absent GFCI protection to be safely used to provide electrical power to the electrical system of a home or other building during utility power interruption. More particularly, the interlock arrangement includes structure to prevent access to the duplex receptacle when the electrical generator is coupled to the power cord used to transfer electrical power from the generator to the electrical system of the home. Thus, the duplex receptacles cannot be used to power tools or other electrical devices when the generator is connected to supply power to the electrical system of the home or building. However, when the generator is physically disconnected from the electrical system of the home or building, the interlock arrangement exposes the duplex receptacles thereby enabling their use.
The invention also provides structure to switch an electrical generator between a bonded neutral arrangement and a floating neutral arrangement. Moreover, the structure prevents the simultaneous availability of electrical power at both the multipole locking (or similar) receptacle used to connect the electrical generator to the electrical system of the home or building and the duplex receptacle(s). In this regard, the invention provides structure that unties the generator neutral and generator ground when the electrical generator is being used to supply backup or emergency electrical power to a home or building through its electrical system and ties the generator neutral and the generator ground when the electrical generator is disconnected from the electrical system of the home or building.
It is therefore an object of the invention to provide an interlock arrangement for use with an electrical generator that expands the usability of the electrical generator without requiring user modification of the internal wiring of the electrical generator.
It is another object of the invention to provide an interlock arrangement that prevents use of duplex receptacles of the generator when the generator is connected to provide emergency or backup power to a home or building.
It is a further object of the invention to provide an electrical generator switchably usable as a floating neutral generator or a bonded neutral generator.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description of the drawing figures.